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Abstract

Breaking waves dissipate energy, transfer momentum from the wind to surface
currents and breaking enhances the transfer of gas and mass across the air-sea interface.
Breaking waves are believed to be the dominant source of sea surface sound at
frequencies greater than 500 Hz and the presence of breaking waves on the ocean surface
has been shown to enhance the scattering of microwave radiation. Previous studies have
shown that breaking waves can be detected by measuring the microwave backscatter and
acoustic radiation from breaking waves. However, these techniques have not yet proven
effective for studying the dynamics of breaking. The primary motivation for the research
presented in this thesis was to determine whether measurements of the sound generated
by breaking waves could be used to quantitatively study the dynamics of the breaking
process.
Laboratory measurements of the microwave backscatter and acoustic radiation
from two-dimensional breaking waves are described in Chapter 2. The major findings of
this Chapter are: 1) the mean square acoustic pressure and backscattered microwave
power correlate with the wave slope and dissipation for waves of moderate slope, 2) the
mean square acoustic pressure and backscattered microwave power correlate strongly
with each other, and 3) the amount of acoustic energy radiated by an individual breaking
event scaled with the amount of mechanical energy dissipated by breaking. The
observed correlations with the mean square acoustic pressure are only relevant for
frequencies greater than 2200 Hz because lower frequencies were below the first acoustic
cut-off frequency of the wave channel.
In order to study the lower frequency sound generated by breaking waves another
series of two-dimensional breaking experiments was conducted. Sound at frequencies as
low as 20 Hz was observed and the mean square acoustic pressure in the frequency band
from 20 Hz-l kHz correlated strongly with the wave slope and dissipation. A
characteristic low frequency signal was observed immediately following the impact of
the plunging wave crest. The origin of this low frequency signal was found to be the
pulsating cylinders of air which are entrained by the plunging waves. The pulsation
frequency correlated with both the wave slope and dissipation. Following the
characteristic constant frequency signal, approximately 0.25 s after the initial impact of
the plunging crest, another low frequency signal was typically observed. These signals
were generally lower in frequency initially and then increased in frequency as time
progressed.
To determine if three-dimensional effects were important in the sound generation
process and to measure the sound beneath larger breaking waves a series of experiments
was conducted in a large multi-paddle wave basin. Three-dimensional breaking waves
were generated and the sound produced by breaking was measured in the frequency
range from 10 Hz to 20 kHz. The observed sound spectra showed significant increases in
level across the entire bandwidth from 10 Hz to 20 kHz and the spectra sloped at -5 to-6
dB per octave at frequencies greater than 1 kHz. The mean square acoustic pressure in
the frequency band from 10 Hz to 150 Hz correlated with the wave amplitude similar to
the results obtained in the two-dimensional breaking experiments. Large amplitude low
frequency spectral peaks were observed approximately 0.75 s after the initial impact of
the plunging crests.
It was postulated that the low frequency signals observed some time after the
initial impact of the plunging crests for both the two and three-dimensional breakers were
caused by the collective oscillation of bubble clouds. Void fraction measurements taken
by Eric Lamarre were available for five breaking events and therefore the average sound
speed inside the bubble clouds and their radii were known. Using this information the
resonant frequencies of a two-dimensional cylindrical bubble cloud of equal radius and
sound speed were calculated. The frequencies of the observed signals matched closely
with the calculated resonant frequencies of the first and second mode of the two-dimensional
cylindrical bubble cloud. The close agreement supports the hypothesis that
the low frequency signals were produced by the collective oscillation of bubble clouds.
In Chapter 4 a model of the sound produced by breaking waves is presented
which uses the sound radiated by a single bubble oscillating at its linear resonant
frequency and the bubble size distribution to estimate the sound spectrum. The model
generates a damped sinusiodal pulse for every bubble formed, as calculated from the
bubble size distribution. If the range to the receiver is known then the only unknown
parameters are ε, the initial fractional amplitude of the bubble oscillation and L, the
dipole moment arm or twice the depth of the bubble below the free surface. It was found
that if the product εxL is independent of the bubble radius the model reproduces the
shape and magnitude of the observed sound spectrum accurately. The success of the
model implies that it may be possible to calculate the bubble size distribution from the
sound spectrum. The model was validated using data from experiments where the
breaking events were small scale gently spilling waves (Medwin and Daniel, 1990).

Description

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1991

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